The present invention generally relates to a station selecting apparatus which has at least two local oscillators, such as an up/down tuner station selecting apparatus and so on to be used in, for example, a CATV and so on.
Generally, in, for example, a CATV in America, since the frequency zone thereof ranges to 54 through 440 MHz, a station selecting apparatus of a double superheterodyne type is normally used. The station selecting apparatus of a double superheterodyne type is also called an up/down tuner. It converts a received high-frequency signal into a first intermediate frequency signal by the conversion from the upper side station so as to attenuate the image interference signal, and thereafter, it converts it into a second intermediate frequency signal free from the image interferences by the conversion from the lower side station. The basic construction of such a station selecting apparatus is shown in FIG. 2.
One example of the conventional station selecting apparatus will be described hereinafter in accordance with the drawings.
FIG. 2 is a block diagram of the conventional station selecting apparatus.
Television signals, for example, VHF or UHF inputted from the input terminal a are amplified by a pre-amplifier 1 and are fed into a first mixer 2. The first mixer 2 mixes the input television signal with the oscillation output signal of the first local oscillator 6. The frequency is converted by the conversion from the upper side station into the first intermediate frequency such as a first intermediate frequency signal of 965 MHz, and is fed into the first intermediate frequency amplifier 3.
The first local oscillator 6 is composed of a voltage control type of oscillator (VCO) of, for example, 1 through 2 GH in oscillation frequency. The oscillation output signal is divided in frequency by a pre-frequency-divider 12. The frequency division output signal is inputted into a variable frequency divider 13 from the terminal b, and is variably divided in frequency by the variable frequency divider 13. The first control signal, which contains the channel information, is added to the variable frequency divider 13 by a microcomputer (not shown). The variable frequency divider 13 changes its frequency division ratio in accordance with the first control signal. The output signal of the variable frequency divider 13 is added to a phase comparator 10 as one input signal. The output signal with the reference oscillation frequency signal caused by the crystal oscillation circuit 8 being divided in frequency by the reference frequency divider 9, and the output signal of the variable frequency divider 13 are compared in phase by the phase comparator 10. The detection output signal of a phase comparator 10 is inputted into the first local oscillator 6 from the terminal c through a loss-pass filter 11, and is controlled in frequency. Namely, the oscillation frequency of the first local oscillator 6 is controlled the oscillation frequency in accordance with the channel information. The frequency division ratio of the variable frequency divider 13 is set so that the oscillation frequency of the first local oscillator 6 may become higher by the first intermediate frequency than the frequency of the input television signal.
The output signal of the first mixer 2 is amplified by the first intermediate amplifier 3, and has the image interference removed, and is fed into the second mixer. It is mixed with the oscillation output signal of the second local oscillator 7, and is converted into the second intermediate frequency signal of, for example, 58.75 MHz by the conversion from the lower side station. The signal from the mixer 4 is inputted into the second intermediate frequency amplifier 5, is amplified, and thereafter, is outputted from the terminal e. Also, the second intermediate frequency signal which is the output of the second intermediate frequency amplifier 5 is fed into the AFC (Automatic Frequency Control) detector 14 from the terminal e. Either one of the detection output signals of the AFC detector 14 is selected with respect to the fixed voltage V by the switch circuit 16, and is inputted into the second local oscillator 7 from the terminal d through the low-pass filter 15, and is controls the oscillation frequency of the second local oscillator 7. The second control signal is adapted to control a switch circuit 16. When the power supply has been turned on or off or the channel has been changed, the fixed voltage V is outputted into the low-pass filter 15. Except for it, the output signal of the AFC detector 14 is adapted to be outputted into the low-pass filter 15. This is a known art of preventing the operation errors at the channel switching time or the like.
Namely, in the circuit of FIG. 2, by the first local oscillator 6 and the first mixer 2 composing a PLL (Phase Locked Loop) circuit, the first control signal is changed so as to change the frequency of the signal from the first local oscillator 6; a high frequency signal to be converted into the first intermediate frequency signal is selected to be tuned into the signal of a particular frequency (television signal of the channel to be selected) to have the channel selecting station and also, to effect the up conversion. The down conversion is effected by the second local oscillator 7 and the second mixer 4 for the operation of the AFC function. It is to be noted that in FIG. 2, the block surrounded with the broken points show a tuner housing member.
In the circuit of FIG. 2, there may be a case wherein the television signal may be inputted in a condition where it is shifted from the normal frequency. In this case, the oscillation frequency of the first local oscillator has to be controlled so that the frequency of the output signal of the first mixer 2 may become 965 MHZ.
A detuning resolution frequency of the first local oscillator 6 is determined by the product of a fixed resolution (frequency division ratio) and the output signal frequency of the reference frequency divider 9. Assume that the frequency division ratio of the pre-frequency-divider 12 is 1/64, the frequency of the output signal of the reference frequency divider 9 is 1 KHz, and the detuning resolution frequency of the first local oscillator 6, namely, the variation in the oscillation frequency due to the change by one step of the frequency division ratio in the movable frequency divider 13, becomes 64 KHz. Therefore, the first local oscillator 6 changes in frequency at 64 KHz steps, so that a sufficient fine adjustment cannot be effected in the channel setting.
It is considered to set the output signal frequency of the reference frequency divider 9 at a lower frequency in order to make the detuning resolution frequency smaller. But the capacity of the capacitor of the low-pass filter 11 must then be larger, with a problem that the cost of the low-pass filter 11 becomes higher and the response characteristics become worse.